Manufacture of cellulosic products



M rch 6, 1962 D. H. GRANGAARD ETAL 3,024,158

MANUFACTURE OF CELLULOSIC PRODUCTS Filed July 2, 1958 MIXING 10 CHEST 2 Sheets-Sheet 1 ADD WATER T0 CONSISTENCY 0F BETWEEN 2 AND IS% ADD NQHCO; 0R SUITABLE BUFFER T0 OF BETWEEN 7 AND I DILUTED PULP BLEED INERT GASES T LIQUOR Z SIDE STREAM PRESSURE VESSEL 1 92' 21% (MAINTAIN AT TEMP! GAS-LIQUID 0F TRANSFER IOO-IGO'C.) 14 I o, FORTIFIED LIQUOR ADD 0;,

UNDER PRESSURE 0F AT REVERSION RESISTANT LEAST 40* FPS]. TREATED PULP United States Pater 3,024.158 MAN UEAQTURE F CELLULOSIC PRODUCTS Donald H. Grangaard and George H. Saunders, Appleton, Win. assignors to Kimberly-Clark Corporation, Neenah, Wis., a corporation of Delaware Filed July 2, 1958. Ser. No. 746,086 8 Claims. (Cl. 16217) The present invention is concerned with a method of manufacturing cellulosic products and more particularly with a method of obtaining bleached cellulosic products which are highly resistant to brightness reversion.

cellulosic products, and in particular wood pulps, usually have a low visual whiteness either because of the presence of coloring materials or other materials which tend to lower the reflectance of the pulp for white light. A substantial proportion of all cellulosic products are therefore bleached to improve the brightness of the product. The term bri htness is used by the pulp and paper industry to indicate the reflectance of a cellulosic pulp or paper product. The brightness may be measured by means of a reflection meter in accordance with the Technical Association of the Pulp and Paper Industry (TAPPI) Method T452-M-48. In this method the pulp or paper product is subjected to a beam of monochromatic light having a wave length of approximately 457 millimicrons and the reflectance from the sample is measured. The numerical value obtained, which is a percentage of the reflectance of a similar beam from a standard white surface, is usually referred to as the brightness of the product.

Bleaching is accomplished by treating the cellulosic product with a chemical agent, which in the case of wood puln destroys or solubilizes the coloring matter and the residual lignin material. The most commonly used bleaching agents are chlorine and its compounds such as hypochlorous acid. hypochlcrite salts, chlorine dioxide and sodium chlorite. The bleaching may be a single stage operation or may be a multi-stage operation such as for example the commonly used sequence of bleaching stages in which the pulp is treated as follows:

( 1) An acidic chlorination stage,

( 2) Alkali extraction, I

( 3) High density hypochlorite bleach,

(4) Alkali extraction, and

(5) A final bleaching with chlorine dioxide or other bleaching agent which has a relatively slight tendency to attack the cellulose.

Because bleaching chemicals which are used to solubilize the lignin and the coloring matter also have a tendency to react with the cellulose one of the problems in bleaching is minimizing these undesirable reactions.

Another problem involved in the bleaching of cellulosic materials and one with which the invention is particularly concerned is that of brightness reversion. There is a tendency for bleached cellulosic products to revert to a lower brightness as these products age. This tendency is greatly accelerated if the cellulosic product is subjected to heat and moisture. A test for brightness reversion is based. upon the change in brightness of a sheet made from the pulp as a result of autoclaving the sheet for thirty minutes at a steam pressure of fifteen pounds per square inch. The result may be expressed by means of the reversion factor Rf, according to the formula brightness reversion tests is discussed in TAPPI, vol. 38,

3,624,158 Patented Mar. 6, 1962 No. 10,625-634. Sanitary cellulosic products which are sterilized by autoclaving are very subject to reversion. Reversion is a particularly undesirable characteristic in sanitary products Where the maximum whiteness is desired.

It is an object of the present invention to provide a method of treating cellulosic products to provide a bleached cellulosic product highly resistant to brightness reversion. Other objects of the present invention will be apparent from the following description and drawings.

FIGURE 1 is a flow diagram illustrating an embodiment of the process of this invention employing a batch type treatment.

FIGURE 2 is a diagrammatic view of apparatus employed to carry out the process shown in FIGURE 1.

FiGURE 3 is a flow diagram illustrating another embodiment of the process employing a continuous treatment.

it has been found in accordance with the process of the present invention that cellulosic products can be made highly resistant to reversion by treatment of the material suspended in an aqueous solution, with oxygen under controlledconditions of time, temperature, pH and the conditions affecting the transfer of oxygen from a gaseous phase to an aqueous phase particularly, the oxygen partial pressure, the ratio of oxygen partial pressure to solution vapor pressure and the ratio of the area of the gaseous phase-aqueous phase interface to the solution volume. Broadly, the process of the present invention comprises the treatment of cellulosic materials suspended in an aqueous solution maintained at a pH in the range of about 7 to 9 by a suitable buffering agent, at temperatures of the order of lOl) to 160 C., for periods of time of the order of about five to one hundred and eighty minutes. During the period of the reaction time the solution is maintained in contact with an oxygen containing atmosphere in which the partial pressure of the oxygen gas is at least about 40 pounds per square inch, the ratio of the oxygen partial pressure to the vapor pressure of the solution is at least about 0.35 and the ratio of a/v is greater than about four, where a is the total area of the gas-solution interface area in square feet and v is the volume of the solution in cubic feet. This treatment is particularly applicable to cellulosic pulps and will produce a pulp having a reversion factor which may be as small as one-tenth that of the reversion factor of conventionally treated bleached cellulose pulps. In addition to producing a reversion resistant pulp this treatment may also increase the brightness of certain pulps.

In the embodiment of the invention illustrated in FIG- URES l and 2, pulp obtained by chemical or semi-chemical pulping methods is diluted with water to a consistency of between 2 and 15% in mixing chest 10. The diluted pulp slurry is brought to a pH of between 7 and 9 by addition of a suitable buffer, such as sodium bicarbonate, and delivered to pressure digester 12 through opening 11. The digester 12 is then closed and the pulp mixture 16 heated to a temperature of between and C. While maintaining the temperature in the above range, oxygen is admitted through pressure valve 17 under a pressure of at least 40 pounds per square inch. The oxygen is absorbed by the aqueous liquor flowing through packed absorption tower 14 and the oxygen fortified liquor is circulated through digester 12 by pump 18. A side stream of the reaction solution is continuously removed through screen 13 and pipe 15 and delivered to spray ring 19 located near the top of the packed tower M to be refortiiied. While the recirculated liquor percolates downward through packed tower 14- and perforated plate 2.6, it is again fortified with oxygen. The treatment is continued for between about 5 to minutes. After treatment, pressure is relieved through valve Zli and the treated pulp discharged through valve 21 into stock chest 22.

The flow digaram of FIG. 3 illustrates a continuous process. Pulp is introduced into mixing chest 10, diluted to a consistency of between 2 and 15% and buffered to a pH of from about 7 and 9. The diluted pulp mixture is passed through heat exchanger 23, raised to a temperature of between about 100 and 160 C., and continuously fed through turbo mixer 24- while oxygen under a pressure of at least about 40 pounds per square inch is introduced therein. The treated pulp is then discharged into stock chest 22.

The process of the present invention is generally applicable to the treatment of cellulosic materials such as fibers, threads, cloths, pulps, etc. It has particular applicability to the treatment of pulps which have been obtained from paper making cellulosic raw materials such as wood, flax, bagasse, cotton, bamboo, esparto, straw, hemp, etc., by chemical and semi-chemical pulping methods. While the process may be applied to pulps of low brightness such as unbleached kraft it is most suitably used with pulps which have a substantial degree of brightness. Thus it may be used with the bleached kraft and the bleached sulfite pulps which have been treated with conventional bleaching agents such as chlorine and its derivatives to a GE. brightness of the order of 60 to 80 percent as measured by TAPPI Method No. 452. Since the process of the present invention does impart a certain amount of brightness to the pulps the process may be used as a bleaching step, either alone or to replace the final step of a conventional multi-stage bleaching operation, to obtain a pulp having the desired brightness characteristics and reversion resistance.

The pulp is prepared for treatment by slurrying it in an aqueous solution. The pulp should be diluted with a suflicient amount of water so that it can be readily handled. The suspension should also be sufficiently diluted so that the oxygenated water can readily penetrate to all portions thereof. Any large excess of water, however, over that required to obtain the desired results requires a larger reaction vessel without any improved results being obtained thereby. A preferred consistency is about 5 percent. The term consistency is here used to indicate the percentage of wood pulp present in the wood pulp slurry by weight. Other consistencies, however, in the order of 2 to percent may be employed in suitable equipment.

The hydrogen ion concentration of the reaction mixture must be maintained within a pH range of about 7 to 9 during the major portion of the reaction time. This is preferably accomplished by the use of a buffer in the solution which can be added at the start of the reaction period and will maintain a pH within the correct range throughout the entire period. Although sodium bicarbonate is believed to be the most inexpensive and generally satisfactory agent, other buffering agents capable of maintaining the pH within the desired range may also be used.

The reaction may be carried out between temperatures of approximately 100 and 160 C. At temperatures greater than about 160 C., the cellulose pulp tends to be degraded whereas at temperatures below about 100 C., an excessive reaction time is required. The preferred temperature range is l-l40 C. The reaction time will vary with the other conditions employed. A variable which particularly affects the reaction rate is the temperature of the reaction solution. The rate at which oxygen is transferred from the atmosphere to the solution, and the rate of absorption of oxygen from the solution by the solid materials therein are also of considerable importance. The variables may be adjusted so that the reaction time may be as little as five minutes or as long as three hours. There may be an optimum time for particular pulps and reaction conditions at which maximum reversion resistance is obtained, but this Will vary with the pulp and the conditions employed. A higher temperature for the reaction mixture apparently has a greater effect upon reducing the reversion factor than does an increase in the oxygen partial pressure. The excellent results when the very short reaction times are employed, demonstrate the suitability of the present process for employment in continuous processing equipment. The process of the present invention may, however, be employed in conventional batch type reaction equipment.

There are two factors involved in obtaining adequate transfer of oxygen from the atmosphere in contact with the reaction solution to the reaction solution. The first of these factors is the partial pressure of the oxygen in contact with the reaction solution. It has been found that the partial pressure of the oxygen should be at least about 40 pounds per square inch. Now in addition to the mini mum oxygen partial pressure in the atmosphere, the ratio of the oxygen partial pressure in the atmosphere in contact with the solution to the vapor pressure of the solution at the reaction temperature should be at least about 0.35 and preferably 0.50 or more. Thus if the reaction is carried out with the reaction solution at a temperature of 160 C., so that the solution has an absolute vapor pressure of approximately pounds per square inch the absolute oxygen pressure should preferably be at least about 45 pounds per square inch.

The ratio of the surface area of the solution in contact with the oxygen containing atmosphere, to the volume of the solution is of very considerable importance in obtaining the desired results with the present process. In general, this ratio of area in square feet to volume in cubic feet must be greater than about 4.0. The term solution volume is used throughout the present specification and claims to include the total volume occupied by the reaction solution and solids suspended therein both within the reaction vessel and in any re-entrant side stream and may be referred to as v. Batch reaction vessels are usually of the cylindrical or spherical type and are designed to have a relatively small gaseous or vapor atmosphere above the reaction mixture. The ratio of the continuous gas-liquid interfacial area in square feet to liquid volume in cubic feet in such vessels is usually quite low. Such vessels, however, may be used to carry out the reaction of the present invention it modifications in the operation are made to obtain the desired interfacial area to solution volume.

One method comprises the passing of oxygen gas through the solution during the reaction period. The oxygen may be introduced at or near the bottom of the reaction vessel so that it bubbles through the reaction solution throughout the reaction period. This establishes a discontinuous gas-liquid interfacial area in addition to the continuous interfacial area which greatly increases the total gas-liquid interfacial area to solution volume ratio. Fresh oxygen from an external source may be passed through the solution or oxygen from the gas space in the dome of the reaction vessel above the solution may be passed through the solution or a combination of the two methods may be used. Agitation should be provided so that uniform mixing of the oxygen with the reaction solution is maintained. The oxygen is preferably introduced through distributor nozzles or'with agitation so that the gas bubbles achieve a minimum size and are evenly dispersed throughout the solution. Modern methods of simultaneously mixing a gas in a liquid and producing liquid shear in reaction vessels, for example by turbo mixing, can also advantageously be employed.

Where the reaction is carried out in a batch type reactor a portion of the reaction solution may be removed as a side stream from the main body of the solution in the reaction vessel and passed through a gas-liquid transfer apparatus of some variety such that it is brought in contact with oxygen under conditions of very large surface area to volume ratio. This side stream may then be returned to the main body of the reaction solution. The

choice of gas-liquid contactor apparatus, of course, must be suitable for the consistency of the pulp-water slurry which is being treated. Where the consistency is not too great, apparatus of the bubble cap tower type or the falling film type may be employed. Alternatively, the pulp may be removed from the side stream, for example by screening as it leaves the reaction vessel so that a clear solution is passed through the gas-liquid contactor.

The solubility of the oxygen in the solution is not appreciably affected by the presence of other indifferent gases in the system. Therefore, not only pure oxygen can be used in the system but any oxygen containing gas in which the diluent gas or gases are inert can be used in the gas transfer apparatus and reaction vessel. However, the minimum partial pressure of the oxygen must be maintained within the limits specified in the present specification. It is therefore desirable to use a relatively concentrated oxygen-atmosphere since the total atmospheric pressure can thus be maintained at the lowest possible figure during the reaction period. The inert gases may be removed during the reaction from the gas space in the reaction vessel in order to maintain the total pressure in the reaction vessel at a minimum.

The term continuous gas-solution interfacial area is used throughout the present specification and claims to describe the surface of the main body of the reaction solution which is in continuous static contact with the oxygen atmosphere at the top of the reaction vessel. The term discontinuous gas-solution interfacial area refers to all other surfaces of the reaction solution which are exposed dynamically to an oxygen atmosphere, for example, the reaction solution surface exposed to bubbles of oxygen which are passed through the main body of reaction solution in the reaction vessel, or the surface of a side stream of reaction solution from the reaction vessel which is exposed to a gaseous atmosphere in a vaporliquid transfer apparatus separate from the reaction vessel. The term side stream refers to the stream of reactor solution which is removed from the main body of the solution and then returned to the main body of solution after passing through a gas transfer apparatus of some variety. The term total gas-solution interfacial area refers to the sum of the continuous and discontinuous interfacial areas. This area is designated as a in the present specification and claims.

Where the process of the present invention is employed as a continuous process the apparatus employed is preferably one in which the oxygen containing gas is passed concurrent to the reaction mixture. Countercurrent processes may also be employed. The reaction may be accomplished in conventional vapor-liquid transfer apparatus such a spray towers, wettecl wall columns, perforated plate towers, bubble cap plate towers, etc. All such methods of vapor-liquid transfer are based in part upon obtaining a ratio of gas-liquid interfacial area to liquid volume which is at least adequate for practicing the process of the present invention.

Now that the process of the invention has been generally described it may be further illustrated by the examples which follow.

The first series of examples which are tabulated in Table I illustrate the application of tne process of the present invention to unbleached pulps obtained by the chemical pulping of wood chips. Examples l-4 inclusive are a kraft pulp having a brightness before treatment of 25.8 percent (R =0.258) as measured on the General Electric brightness meter in accordance with TAPPI Standard No. 452M-48. The brightness measured on a General Electric brightness meter is usually referred to as GE. brightness and is the percent reflectance compared to a standard. The brightness may also be expressed as a decimal of the reflectance of a sheet or sheets sufiicient in thickness to prevent transmission of any of the incident light, for example 12 :0258 and will be so expressed in the present examples. The pulp which was treated in Examples 5-8 inclusive, was an unbleached pulp obtained by the sulfite process and having an original GE. brightness of 51.6 percent (R =0.516). In each of the examples tabulated in Table I the pulp was diluted with water to a consistency of approximately 2.5 percent. The term consistency is used throughout the present specification to mean the percentage by weight of air dry pulp in any combination of pulp and water. Air dried pulp is understood to contain 10 percent moisture. To this reaction mixture was then added the amount of sodium bicarbonate shown in the table to buffer the reaction mixture to a pH between 7 and 9. The reaction mixture was then placed in a reaction vessel, the vessel closed and the reaction mixture heated to a temperature of C. Oxygen was then admitted to the reaction vessel so that the oxygen partial pressure in the vessel was approximately 162.3 psi. The ratio of the oxygen partial pressure to the partial pressure of vapor was thus approximately three to one. The mixture was so agitated during the reaction period that the ratio of the area of the interface between the gas atmosphere and the reaction mixture in square feet to the reaction mixture volume in cubic feet was substantially greater than four throughout the reaction period. The reaction periods are shown in the table. Upon completion of the reaction period the reaction mixture Was removed from the reaction vessel and washed. Samples of the pulp were then made into hand sheets in accordance with TAPPI Standard T-218 and the reflectance determined with a GE. brightness meter in accordance with TAPPI Standard T452M48. The hand sheets were then placed in the steam sterilizer and sterilized at fifteen pounds per square inch gauge pressure for thirty minutes. The sterilizer was then cooled under vacuum for twenty minutes and the hand sheets removed from the sterilizer and the brightness values redetermined. The brightness values before and after sterilization are shown in the table. The reversion factor was then determined in accordance with the formula In the formula R1 is the reversion factor, k is the specific absorption coefiicient of the pulp before sterilization; k is the specific absorption coefticient of the pulp after sterilization and s is the specific scattering coeificient. The specific scattering coefficient is assumed to remain unchanged during the sterilization. The k/s value corresponding to the reflectance PM may be determined by reference to TATPI Data Sheet 65A (which is also printed as Table I in an article by MacLaurin and Afienzer TAPPI, vol. 37, No. 9, September 1954, 3384393).

TABLE I Reflectance R Rever- Example NaHC 0 B Ti'ne, siou min. Factor Before b After Rf B Parts NZIHGOQ, per ten parts air dry pulp.

b Reflectance measured before and after sterilization of hand sheets.

As shown by the above table even unbleached pulp treated by the present process has comparatively low reversion factors compared to untreated pulp. it will also be noted that a substantial brightness increase is obtained by the process.

The second group of examples tabulated in Table II iilustrates the effect of the process of the present invention when employed with conventionally bleached pulps.

The pulp employed in Examples 9-11 was a hypochlorite bleached kraft pulp; the pulp employed in Examples 12-14 was a chlorine dioxide bleached kraft pulp and the pulp employed in Examples 1517 was a second hypochlorite bleached kraft pulp. The pulps of Examples 9, 12 and 15 were not treated by the process of the present invention but are examples of the pulp upon which the reversion tests were run for comparison. In Examples 10, 11, 13, 14, 16 and 17 samples of the pulp were diluted with water to a consistency of 2.5 percent. Two parts of Nat- CO was then added to the pulp slurry per ten parts of air dried pulp. In each example the resulting reaction mixture was then placed in a reaction vessel which was then heated to a temperature of 140 C. Upon obtaining this temperature the oxygen was added to the reaction vessel to a partial oxygen pressure of approximately one hundred and sixty-two pounds per square inch. The other reaction conditions were the same as in Examples 18. Upon completion of the reaction period as shown in Table II the pulp was removed, washed, made into hand sheets and the reversion factor determined as in Examples 18. The reflectance values and reversion factors are shown in Table II.

As shown by the table, the process of the present invention affords very substantial improvements in the re version characteristics of bleached pulps treated by the present method. It may also be noted that the process of the present invention may increase the brightness of even bleached pulps as well as improving the reversion characteristics.

The third group of examples tabulated in Table III illustrates variations in conditions of time, temperature and oxygen partial pressure. These examples, particularly Examples 21, 23 and 28, illustrate the adaptability of the present process to continuous processing methods. All examples of Table III were carried out with the same pulp, a bleached kraft pulp, having an initial G.E. brightness of 74.14 percent, equivalent to a reflectance of 0.741. Example 18 is a blank showing the reversion factor of the untreated pulp. In the other examples tabulated in Table III a sample of the pulp was diluted to a consistency of five percent and to the resulting mixture was added five parts of sodium bicarbonate per twenty parts of air dry pulp to bring the pH of the mixture to within the range of 7 to 9. In each case the resultant mixture was then placed in the reaction vessel and the temperature of the reaction mixture brought up to the temperature indicated in the table. Oxygen was then added to the reaction vessel to approximately the partial pressure indicated in the table and the reaction carried out with the other conditions the same as described in Examples 18, for the period of time indicated in the table. The pulp after removal from the reaction vessel was washed and the reversion factor determined in accordance with the method described in connection with Examples 1-8. The reflectance before and after sterilization and the reversion factor of each sample is shown in the table'.

TABLE III Oxygen Reflectance Example partial Time, Temp, Reversion pressure, rnin. factor p.s.i. Before After As shown by the above table, there is a very substantial decrease in the reversion factor with the process of the present invention even when very short reaction periods of the order of five minutes are employed.

The following example illustrates the process of the present invention as applied to a cotton cloth.

Example 28 An unbleached cotton gauze having a brightness of 50.4 percent as measured on a General Electric brightness meter was selected. Twenty parts of the gauze was suspended in four hundred parts of a sodium bi carbonate solution containing five parts of sodium bicarbonate. The solution containing the gauze was then placed in a reaction vessel having an oxygen partial pressure of the atmosphere above the solution of seven hundred and fifty pounds per square inch. The reaction vessel was brought up to a temperature of C., and maintained at that temperature for one hour while the interfacial area to solution volume was maintained above four. Upon completion of the reaction period the gauze was tested for brightness and was found to have a GE. brightness of 75.9 percent. The gauze was then autoclaved at a pressure of fifteen pounds per square inch gauge. The brightness after autoclaving was 64 percent. The reversion factor was thus 50.8. The gauze was also tested for absorbency before and after the treatment by means of a sink test. In this test a pad of thirty-two plies of the gauze was placed on the surface of water at a temperature of 70 F., and the time lapse between the pads first contact with the water and the time the pad becomes completely wetted, measured. The sink time of the gauze before treatment was thirty minutes and the sink time after treatment was three seconds.

Now that the process of the present invention has been described it is believed that it will be apparent that numerous modifications can be made without departing from the scope of the present invention which is to be defined only by the claims which follow.

What is claimed is:

1.'A method of improving the resistance of bleached cellulosic products to brightness reversion which comprises treating cellulosic fibers obtained from chemical and semi-chemical pulping processes in an aqueous solution maintained at a consistency of between about 2% and 15% and a pH of from about 7 to 9, at a temperature of from 100-160" C., with a gaseous oxygen atmosphere containing oxygen under a partial pressure of at least about forty pounds per square inch, for from about five to one hundred and eighty minutes under conditions such that the ratio of oxygen partial pressure to vapor pressure of the solution is at least 0.35 and the ratio of 11/11 is greater than about four, Where a is the total gas-solution interfacial area in square feet and v is the solution volume in cubic feet.

2. In a process for improving resistance of bleached wood pulp to brightness reversion, the method which comprises treating bleached wcod pulp in an aqueous solution maintained at a consistency of between about 2% and 15% and a pH of from about 7 to 9, at a temperature of 120l40 C., for from about five to one hundred and eighty minutes, with a gaseous oxygen atmosphere under a partial oxygen pressure of at least about forty pounds per square inch, under conditions such that the ratio of oxygen partial pressure to vapor pressure of the solution is at least about 0.50 and the ratio of a/ v is greater than about four, where a is the total gas-solution interfacial area in square feet and v is the solution volume in cubic feet.

3. The process of claim 2 wherein the pH is maintained between about 7 and 9 by a sodium bicarbonate buffer.

4. The process of claim 2 wherein the interfacial area to solution volume ratio is maintained by continuously passing oxygen gas in finely dispersed form through the solution.

5. The process of claim 2 wherein the interfacial area to solution volume ratio is maintained by treating a side stream of said solution with oxygen.

6. In a continuous process for treating fibers to improve resistance to brightness reversion, the method which comprises suspending cellulosic fibers obtained from chemical and semi-chemical pulping processes in an aqueous solution maintained at a consistency of between about 2% and 15% and a pH of about 7 to 9, passing a continuous stream of said solution through a reaction zone maintained at a temperature of about 100-160 C., at a rate such that the solution is maintained in the zone at least about five minutes, said reaction zone having an oxygen atmosphere wherein the partial oxygen pressure is at least pounds per square inch and the ratio of the partial pres sure of oxygen to the vapor pressure of the solution is at least about 0.35 and the interfacial atmosphere-solution ratio of a/v is greater than about four, where a is the total gas-solution interfacial area in square feet and v is the solution volume in cubic feet.

7. The process of claim 6 wherein the stream of the solution is passed through the reaction zone countercurrent to a flow of oxygen gas.

8. The process of claim 6 wherein the stream of the solution is passed through the reaction zone concurrent to a flow of oxygen gas.

References Cited in the file of this patent UNITED STATES PATENTS 1,163,438 Muller Dec. 7, 1915 1,224,145 Craig May 1, 1917 2,673,148 Harris Mar. 23, 1954 2,811,518 Mitchell Oct. 29, 1957 2,926,114 Grangaard Feb. 23, 1960 

1. A METHOD OF IMPROVING THE RESISTANCE OF BLEACHED CELLULOSIC PRODUCTS TO BRIGHTNESS REVERSION WHICH COMPRISES TREATING CELLULOSIC FIBERS OBTAINED FROM CHEMICAL AND SEMI-CHEMICAL PULPING PROCESS IN AN AQUEOUS SOLUTION MAINTAINED AT A CONSISTENCY OF BETWEEN ABOUT 2% AND 15% AND A PH OF FROM ABOUT 7 TO 9, AT A TEMPERATURE OF FROM 100-160* C., WITH A GASEOUS OXYGEN ATMOSPHERE CONTAINING OXYGEN UNDER A PARTICAL PRESSURE OF AT LEAST ABOUT FORTY POUNDS PER SQUARE INCH, FOR FROM ABOUT FIVE TO ONE HUDRED AND EIGHTY MINUTES UNDER CONDITIONS SUCH THAT THE RATIO OF OXYGEN PARTICAL PRESSURE TO VAPOR PRESSURE OF THE SOLUTION IS AT LEAST 0.35 AND THE RATIO OF A/V IS GREATER THAN ABOUT FOUR, WHERE A IS THE TOTAL GAS-SOLUTION INTERFACIAL AREA IN SQUARE FEET AND V IS THE SOLUTION VOLUME IN CUBIC FEET. 